Human males have never been thought of as models of sexual restraint — and with good reason. From the moment the adolescent libido begins to boot up, boys seem to enter an ongoing state of emotional — if not literal — priapism, from which they never fully emerge.

As far as nature is concerned, this is just fine. The goal of any organism, after all, is to ensure the survival and propagation of its genes, and males — far more so than females — are eminently equipped to do that. Even the world’s most reproductively prolific mothers rarely produce more than eight or nine children in a lifetime. Males can conceive everyday, even multiple times a day, and come emotionally hardwired to do just that.

Part of the reason they don’t, apart from the impracticality of trying to raise a brood of 200 children, is that they just don’t get that many mating opportunities. Sex requires a willing partner, and females, with so much more on the line in terms of the time, effort and energy that pregnancy and child-rearing involve, can be extremely selective in choosing mates. That requires males to develop a whole suite of emotional muscles — self-denial, self-restraint, a facility for delayed gratification — that will help them cope with an appetite that at some levels will never be fully satisfied. And that, in turn, is a central pillar of monogamy and fidelity.

But what happens when the lid comes off? What happens when the opportunities are unlimited? In some cases, not much. Males either continue to practice self-control or, after a brief period of happy debauchery — think rock stars or leading men who are always seen stepping out with a new piece of arm candy — finally settle down into stable monogamy.

In some cases, though, the stability never happens; in some cases, unlimited opportunity simply leads to unlimited appetites. Emperors and despots may be best known for this kind of behavior. The 18th-century Moroccan ruler Moulay Ismail is said to have fathered 888 children with his 500 concubines. Genghis Khan makes Ismail look practically barren. A 2003 analysis of the Y chromosome of 2,123 men now living across the former Mongol empire showed that there are 16 million males living today whose line stretches back to the great conqueror — or one out of every 200 males now on the planet. But modern-day men of power — Donald Trump, Bill Clinton, Eliot Spitzer, John Edwards, John Ensign, JFK, FDR, and most recently, Arnold Schwarzenegger and Dominique Strauss-Kahn — with their serial wives or serial philandering, can behave just as badly, if less prolifically.

“When men have more opportunity they tend to act on that opportunity,” says psychologist Mark Held, a private practitioner in the Denver area who specializes in male sexuality and the problems of overachievers. “The challenge becomes developing ways to control the impulses so you don’t get yourself into self-defeating situations.”

There are a lot of things that distinguish pampered and powerful men who behave themselves sexually from those who don’t. Part of it is simply underlying temperament. Bill Clinton and Harry Truman were decidedly different people in the rectitude department, even if both of them were once the most powerful person in the world. There’s no saying with certainty what led to Clinton’s sometimes frantic need to be liked and his maddening lack of any internal governor — his broken home, his abusive stepfather, his admitted issues with low self-esteem that came from being seen as the chubby kid from the school band — but other leaders have hard-knock pasts too and not all of them become lotharios.

“It’s sometimes framed as a case of character, but it can also be someone’s sexual makeup,” says Dr. Fred Berlin, founder of the sexual disorders clinic at Johns Hopkins University. “There’s a basic biological factor at play. Sex is a powerful force and some people just have to struggle more to control it than others.”

Narcissists are clearly part of the poor-control camp. One of the things that made John Edwards a hard electoral sell well before his affair was revealed was the visceral sense people got that he was simply too much of a self-adoring preener. There would be no level of public adoration he could achieve that would reach the level he felt for himself. But in some cases, narcissism is not the only thing going on.

“There’s a fairly new measure we use that’s called ‘the dark side,'” says Larry Josephs, a professor of psychology at Adelphi University. “It’s a combination of narcissism, Machiavellianism and psychopathy.” Garden variety cads like Edwards clearly aren’t psychopathic, but world-class monsters like Caligula — whose sexual appetites were legend — just as clearly were. All three features of the dark side personality can exist in people in different combinations and different degrees, but the more severe any one of them is, the greater the risk for sexual acting out. “Men who are low on these traits are less likely to cheat even when the opportunity presents itself,” says Josephs.

Also in play is the point in life at which unusual privilege was first conferred. Members of royal families are born into a world of indulgence and entitlement, and the princelings who grow up that way may never have to develop the emotional musculature that will allow them to show self-restraint. Athletes often start life at the opposite end of the wealth and prestige spectrum, but as soon as they exhibit an unusual talent for swinging a bat or sinking a free-throw they may find that the rules have been suspended for them. They are waved through school and into the pros, and incidents of bad behavior are overlooked or covered up. Any skills they may have been developing for self-control or self-denial quickly wither.

“People who come into fame or power later in life do seem to manage their circumstances more effectively,” says Berlin. “They’ve learned a certain discipline and they’ve adapted to having to contain themselves.”

Men, certainly, aren’t the only people who abuse their power sexually. Women exhibit the dark-side triad too, and can become accustomed to power and its perks as easily as a man can. What’s more, testosterone, a primal driver of dominance behavior, is not the exclusive province of men either. “Women produce testosterone just like men do, even if at different levels,” says Josephs. “That means women have testosterone-driven tendencies as well, and that pays dividends. Dominant animals tend to be more reproductively successful whether they’re male or female.”

But if alpha men and women dominate both the world and the gene pool, that’s not to say betas don’t have ways to push back. Among all social animals, leaders are tolerated only until what they give back to the group in terms of stability and order is exceeded by what they take from it, in terms of material and mating resources. When the balance tips the wrong way, the group gets restless.

“We have what we call subordinate strategies, and we see that in chimps and bonobos too,” says Josephs. “You don’t want the leader dominating all of the mating opportunities, so you form coalitions and topple him.”

That same phenomenon, Josephs believes, may explain the public outrage when sexual misbehavior of elites — particularly the kind that involves violence or assault — become public. “We don’t want our leaders to be philanderers,” he says. “In an egalitarian society, nobody should monopolize all of the females or sleep with our wives, or we’re going to get even.”

In the case of Edwards or Spitzer, that meant banishment to a political Elba. In the case of Strauss-Kahn, it may well mean jail. In the case of Caligula, it meant assassination. It’s been said for millennia, but it bears repeating: just keep it zipped, boys, just keep it zipped.